This invention relates to a body joint endoprosthesis, and more particularly to an endoprosthesis that transmits forces in a manner more closely approximating normal physiological stress distribution.
Wear phenomena in the joints of aged people, the consequence of inherited disease, rheumatic inflammatory ailments and injuries may lead to chronic pain conditions and progressive restriction of joint mobility (arthrosis), which often severly limits the sphere of activity and imposes acute physical stress upon the sufferer.
By way of showing the state of the art, reference is made to the paper "Technischer Fortschritt bei kunstlichen Huftgelenken" in the periodical Technische Rundschau Sulzer, 4/1974, pages 235 to 245. This treatise gives a review of the development of artificial hip joints and describes typical constructional forms known since 1939. From the discussion in the paper of these particular structural forms, it is apparent that all of the constructions have more of less serious inherent disadvantages. In addition to wear phenomena in the two parts of the joint, the ball and the socket, the known shaft prosthesis are accompanied by the particular disadvantage that loosening of the sockets and the shafts occurs, accompanied in certain cases by the subsequent breakage of the shaft. In the known artificial hip joints, this loosening of the shaft can be attributed to several different causes. Firstly, the shaft is preferably cemented into the bone. The cement which is used is the self-polymerizing synthetic plastic material methyl methacrylate. The heat of polymerization resulting from the curing of this material results in temperatures of 80.degree. to 100.degree. C. and can cause thermal damage of the surrounding tissue, because the coagulation point of albumen amounts to 56.degree. C. In addition to these thermonecroses, damaging effects are also revived which resulted from the mechanical preparation of the bone (rasping or similar operations) carried out during the preparation of the seating to receive the implant, and at the same time there is formed between the damaged area and the bone a screen of connective tissue, which has a negative influence upon the anchorage in the bone. The screen of connective tissue permits micro relative displacements to take place between the implant and the bone seating. For a fuller treatment of this condition, reference is made to the book Biopolymere und Biomechanik von Bindegewebssystemen, Springer-Verlag 1974, pages 417 to 419, in the article "Zur Problematik der Zementverankerung im Knochen" by H. G. Willert.
A further cause of the loosening phenomenon is the nonphysiological nature of the application of force from the implant to the bone. For the want of any other exposition on this subject in the known art, an explanation thereof will now be given with reference to FIGS. 1, 2, and 3 of the accompanying drawings.
In FIG. 1 a shaft 35 of a femur head endoprosthesis 37 is secured in a thigh bone 30 by means of cement 32. The shaft 35 has a collar 39 and a neck 40 extension terminating in a ball 42 having a center point M. The ball 42 rests in a ball socket 45, which is secured in a pelvic bone 48 by cement 46. The resulting force F.sub.R passes through the center point M of the ball 42 and in FIG. 1 is shown in the direction in which this resultant force has its maximum value. In that case, the direction of the force with respect to the longitudinal axis of the neck 40 encloses an angle .alpha.. If this resultant force F.sub.R is reduced to a point A of the seating surface 50 of the thigh bone 30 cooperating with the collar 39, then there will act in a direction normal to the seating surface 50 the force F.sub.R .multidot.cos .alpha. and in the direction of the seating surface 50 the force component F.sub.R .multidot.sin .alpha.. In addition, due to the parallel displacement of the resultant force F.sub.R there is also effective the force couple F.sub.R .multidot.a in accordance with the parallelogram surface 51 shown hatched in FIG. 1. The distance a represents the shortest spacing of the point A from the line of action of the resultant force F.sub.R.
In FIG. 2 there are indicated qualitatively the surface pressures acting upon the thigh bone 30 and resulting from these two force components and the force couple. It is seen that the surface pressure p acting normal to the seating surface 50 is practically constant, while in the physiological case, represented in FIG. 3, the approximately linear curve of the normal stress .sigma. acting at the right hand or medial edge in FIG. 3 shows a maximum compression stress of .sigma..sub.D max. In the physiological case according to FIG. 3 there will be a neutral fiber at a position spaced by a distance e from this position of maximum compression stress so that remote from it at the left hand or lateral edge in FIG. 3 there will be a tensile stress .sigma..sub.Z max.
In the physiological case according to FIG. 3 practically no normal stress arises at right angles to the direction of the fibres of the cortical tissue 122 indicated in FIGS. 1 and 2, but in the other case the force component F.sub.R .multidot.sin .alpha. and the force couple F.sub.R .multidot.a will give rise to the surface pressures q and r, which act at right angles to the inner surface of the cortical tissue. The surface load q at the inner medial margin of the seating surface 50 has a maximum q.sub.max, which, in the course of the continuous reconstitution of the bone, results in a progressive yielding of the bone.
Along with this yielding of the bone, the bending stress of the prosthesis and the cement increases until, due to further loosening of the shaft 35, cracks appear in the cement cladding 32, and finally a breakage 55 of the shaft 35 causes the prosthesis to fail and leads to immobility of the patient. The relative movements between the collar 39 and the seating surface 50 arising from this loosening process prevent the desirable progressive ingrowth of the bone cells into pores, cavities or perforations in the surface of the known prostheses. The above mentioned cracks in the cement cladding lead to intense corrosion phenomena in the metal (crack corrosion). Sinking of the shaft prosthesis is also possible if the support afforded by the cortical tissue is lost.
The force F.sub.R .multidot.cos .alpha. indicated in FIG. 1 as acting at right angles to the seating surface 50 is transmitted through the collar 39, and, in respect of one part, over the seating surface 50 into the thigh bone 30 and, in respect of another part, is transmitted onto this thigh bone by virtue of the positive connection between the implant and the thigh bone 30. If it is assumed that the case under consideration is a known implant of steel, then the relationship of the modulus of elasticity of the steel implant to that of bone is about 8:1. Consequently, under load the bone deformation is relatively greater than that of the steel, and there is a relative displacement of the contacting surfaces of the implant and the bone. These displacements can lead to shearing off of the osteoblasts building up to form a bridge between the bone and the implant, which otherwise are desirable for a lasting anchorage of the implant in the bone. As a result, there is formed in that region a resilient screen of connection tissue, which permits further relative displacements and therefore a loosening of the prosthesis.
In the book entitled Gesammelte Abhandlungen zur funktionellen Anatomie des Bewegungsapparates by Friedrich Pauwels, Springer-Verlag 1965, in particular at pages 4 to 6 of the chapter "Mechanische Faktoren bei der Frakturheilung," a general description is given of the influence of mechanical stimuli upon the final structure of newly formed tissue in the form of connective tissue, cartilage or bone. The force component F.sub.R .multidot.sin .alpha. defined above in connection with FIG. 1 can result in the setting up of a so-called free shearing force in the seating surface, preventing the desirable growth of new bone tissue, this force being explained in the above cited book of Pauwels in the chapter "Die freie Scherkraft" at pages 21 to 24.
Furthermore, attempts are also known to anchor shaft prosthesis in thigh bone without the use of cement. For this purpose the surface of the shaft is provided with perforations or macroscopic depressions, in which it is intended that there shall be a newly formed growth of bone tissue resulting in an intimately contacting anchorage of the shaft of the prosthesis in the thigh bone. Nevertheless, because the implantation of hip joint endoprostheses is mostly necessary in elderly patients, great importance must be placed upon early mobilization of the patient. If the patient is laid up for too long a time until there is a sufficiently firm ingrowth of the shaft of the prosthesis, this delay can, for example, cause the risk of pneumonia, muscular atrophy and damage to the heart and circulation as well as to the bladder and kidney system. For example, in experiments with animals, the time taken for the formation of load bearing bone tissue has been at least two months. Consequently, the known types of prostheses whose anchorage in the thigh bone relies exclusively upon the principle of the ingrowth of bone tissue are therefore very disadvantageous on account of the lack of early mobilization of the patient. Moreover, because of the difference in the moduli of elasticity and locally high surface pressures, loosening phenomena can appear at the implant.
In known hip joint endoprostheses of this type (German Auslegeschrift Specification No. 1,541,246, and French specification No. 2,057,418) a metallic shaft is formed integrally with a collar shaped support element and a bearing stud, upon which a ball head is rotatably mounted as the first part of the joint. There is no joint between the support element and the shaft. In consequence this construction does not remove the above explained disadvantages.
In a further known hip joint endoprosthesis of the above mentioned type (French specification No. 2,210,909) a shaft is formed integrally with a collar shaped support element, to which can be screwed a first joint member in the form of a ball head to form a rigid unit secured by a clamping device. This construction also lacks a joint between the support element and the shaft.
In the Swiss patent specification No. 426,096 there is disclosed a hip joint endoprosthesis, to whose shaft there is integrally connected a first socket. In this socket there is mounted a freely rotatable ball forming the first joint member, which latter is also supported in a socket in the pelvis bone forming the second joint member. There is thus the lack of a support element and accordingly also a lack of the joint according to the present invention. The forces applied to the first socket are transmitted exclusively through the shaft into the thigh bone, so that the abovementioned disadvantages are even more clearly noticeable.
It is already known (German Offenlegungsschrift Specification No. 2,432,766) to construct an artificial knee joint which during the whole of the movement cycle functions as a crossed quadrangular linkage having the bridge connected to the thigh bone and the couple connected to the tibia.